Conventionally, as a material showing a positive PCTR, there has been proposed a composition constituted by adding various semiconductor forming elements to BaTiO3. These compositions are provided with Curie temperature of around 120° C., and therefore, it is necessary to shift the Curie temperature in accordance with the application.
For example, although it has been proposed to shift Curie temperature by adding SrTiO3 to BaTiO3, in this case, the Curie temperature is shifted only in a negative direction and is not shifted in a positive direction. Currently, only PbTiO3 is known as an additive element for shifting Curie temperature in a positive direction. However, PbTiO3 includes an element for bringing about environmental contamination, and therefore, in recent years, a material in which PbTiO3 is not employed has been desired.
In BaTiO3 semiconductor porcelain, with an object of preventing a reduction in a resistance temperature coefficient by Pb substitution, as well as reducing a voltage dependency and promoting a productivity and a reliability, there has been proposed a method of producing a BaTiO3 semiconductor porcelain in which a composition, obtained by adding one or more kinds of any of Nb, Ta and a rare earth element to a composition in which x in Ba1-2X(BiNa)xTiO3 where a portion of Ba of BaTiO3 is substituted by Ba—Na is controlled to a range of 0<x<0.15, is sintered in nitrogen, and thereafter subjected to a heat treatment in an oxidizing atmosphere (see Patent Reference 1).
Patent Reference 1: JP-A-56-169301
In the case in which a valence control of the composition is carried out in a system in which a portion of Ba is substituted by Bi—Na, when a trivalent positive ion is added as a semiconductor forming element, the effect of semiconductor formation is reduced by the presence of monovalent Na ion, and the resistivity at room temperature is increased. Although, in Patent Reference 1, as an embodiment, there has been disclosed a composition constituted by adding 0.1 mol % of Nd2O3 to Ba1-2x(BiNa)TiO3 (0<x≦0.15) as a semiconductor element, the additive amount of Nd2O3 cannot realize semiconductor formation sufficient for PTC application.
In the above-described material, the resistance value thereof is conceived to be caused by Schottky barrier of a grain boundary. As means of controlling the Schottky barrier, oxidation/deoxidation treatment of a grain boundary has been proposed, and it is reported that a high PTC property can be achieved by a material generally subjected to oxidizing treatment in oxygen (see Nonpatent Reference 1). Further, it has been also reported that a treatment rate in the heat treatment also influences on a property (see Nonpatent Reference 2) to pose a problem that the heat treatment of the material becomes very complicated.
Nonpatent Reference 1: Titabari Kenkyukai Shiryo XVII-95-659 (1968)
Nonpatent Reference 2: J. Am. Ceram. Soc. 48, 81 (1965)
Further, according to the heat treatment, there poses a problem that, although the effect by the heat treatment can act uniformly to an inner portion of the material when the shape of the material is a comparatively small, in the case of a comparatively large shape (thick shape) applied in the application such as a PTC heater, it is difficult to provide a PTC property uniform to the inner portion of the material.